1. Field of the Invention
This invention relates to a diamine and a polyamic acid derived from the diamine.
2. Description of the Related Art
Because of lightweight and low power consumption characteristics of a liquid crystal display apparatus, a miniature portable personal information device with a liquid crystal display panel has been widely developed. The arrangement of liquid crystal molecules in a liquid crystal layer of the liquid crystal display apparatus can be changed using an external electric field, thereby adjusting the transmitted amount of incident light. Based on different arrangements of the liquid crystal molecules, liquid crystal display apparatus presently in practical use can be classified into a twisted nematic (TN) liquid crystal display apparatus twisted by 90°, a super twisted nematic (STN) liquid crystal display apparatus twisted by 180° or more, or a liquid crystal display apparatus utilizing thin film transistors (TFT).
In general, TN liquid crystal display apparatus includes, from bottom to top, a first substrate, a first conductive film, a first orienting film, liquid crystal, a second orienting film, a second conductive film, and a second substrate. The first and second orienting films are used to allow liquid crystal molecules to tilt at a pre-tilt angle relative to the first and second substrates. There are two methods for producing the orienting film. The first method is vapor deposition of an inorganic material, such as silicon oxide, on the first and second substrates along an inclined direction so as to form the orienting films on the first and second substrates. The liquid molecules injected after formation of the orienting films can be arranged along the inclined direction. However, this method is not suitable for mass production in the industry. The second method includes forming an organic coating on each of the first and second substrates, and rubbing a surface of the coating along a predetermined direction using a fabric, such as cotton, nylon, or polyester, so as to form well-arranged grooves on the surface of the coating. The liquid crystal molecules injected after forming the grooves are arranged along the grooves. The second method is widely used because of well arrangement of the liquid crystal molecules and easy performance. In the second method, the organic coating can be formed from polyvinyl alcohol, polyoxyethylene, polyamide, or polyimide, in which polyimide is widely used due to high mechanical strength, heat resistance, and solvent resistance.
A conventional method for producing polyimide includes polymerizing a monoamine or a diamine and a tetracarboxylic dianhydride so as to form a polyamic acid, and heating the polyamic acid so as to form a polyimide. To obtain a proper pre-tilt angle (greater than 3°) and good orienting property without deteriorating the property of the polyimide, research has focused on the modification of diamine structure.
U.S. Pat. No. 6,111,059 granted to Nissan Chemical Industries Ltd. discloses a diaminobenzene derivative represented by the general formula (1)

wherein P is a single bond or a bivalent organic group selected from —O—, —COO— and —CONH—, Q is a cyclic substituent selected from an aromatic ring, an aliphatic ring, a hetero ring and combinations thereof, R11 is an aliphatic ring, and R12 is a C1-22 alkyl group having a linear chain. A polyimide is also disclosed in this patent, which is obtained by reacting a diamine containing at least 1 mol % of the diaminobenzene derivative with a tetracarboxylic acid to form a polyimide precursor followed by undergoing a ring-closing reaction. The polyimide has a repeating unit represented by the general formula (2)

wherein A is a tetravalent organic group constituting a tetracarboxylic acid, B is a bivalent organic group constituting a diamine, and P, Q, R11, and R12 are as defined in the formula (1).
Japan Synthetic Rubber Co., Ltd. has proposed diamines and polyimides disclosed in, for example, U.S. Pat. Nos. 5,276,132, 5,698,135, and 5,783,656. U.S. Pat. No. 5,698,135 discloses a liquid crystal-aligning agent including at least one of polyamic acid (referred as “polymer I”) and a polymer (referred as “polymer II”) obtained by imidizing the polymer I. The polyamic acid is obtained by reacting a tetracarboxylic acid dianhydride with a diamine compound. The diamine compound is selected from the group consisting of:
and combinations thereof. In formula (3), R13 is a halogen atom, alkyl, or an alkoxyl group, and a is an integer of 0 to 4. In formula (4), R4 is —CH2—, —O—, —S—, formula (i), or formula (ii), R15 and R16 are independently a halogen atom or an alkyl group, and b and c are each independently an integer of 0 to 4.
In formula (5), R17 is a divalent organic group having a steroidal skeleton. Examples of formula (5) include formulas (6) and (7).

U.S. Pat. No. 6,746,730 granted to Chisso Corporation discloses a varnish composition which includes a polyamic acid B represented by formula (8), a polyamic acid A represented by formula (9), and a soluble polyimide represented by formula (10),
wherein R18, R20, and R22 are independently a tetravalent organic radical derived from tetracarboxylic acids; R19, R21, and R23 are independently a divalent organic radical derived from a diamine compound; and d, e, and f are independently a positive integer. Examples of the diamine compound include formulas (11) to (15).
In formulas (11) to (14), R24, R25, and R26 are independently hydrogen or a linear or branched alkyl group having 1 to 12 carbon atoms, Y is methylene, ring A represents a benzene or a cyclohexane ring, Z represents a single bond, CH2, CH2CH2, or oxygen, r is an integer of 0 to 3, s is an integer of 0 to 5, t is an integer of 0 to 3, provided that when t is 2 or 3, each Z may be the same or different. In formula (15), X1 represents a single bond, CH2, CH2CH2, or oxygen, R27 and R28 are independently hydrogen, or an alkyl or perfluoroalkyl group having a linear or branched alkyl of 1 to 12 carbon atoms, but at least one of R27 and R28 is an alkyl or perfluoroalkyl group having a linear or branched alkyl of 3 or more carbon atoms, and u is an integer of 0 to 3, provided that when u is 2 or 3, each X1 may be the same or different. Although this patent can solve a problem of image sticking and can adjust a pre-tilt angle arbitrarily, the pre-tilt angle merely ranges from 2.7 to 8.9 degree (see Tables 3 and 4 of the patent). In addition, the issue of orienting property of the liquid crystal is not addressed in this patent.
Therefore, there is a need in the art to provide a crystal orienting film that provides a greater range of pre-tilt angles and improved orienting property (i.e., no occurrence of undesired domains).